1. Field of the Invention
The present invention relates to an oven-controlled crystal oscillator that uses a surface-mount type quartz crystal unit, and in particular, to an oven-controlled crystal oscillator that is excellent in responsiveness to a change in temperature.
2. Description of the Related Art
A crystal oscillator includes a crystal unit composed of a quartz crystal blank hermetically encapsulated in a container and an oscillation circuit that uses the crystal unit; the crystal unit and the oscillation circuit are integrated together. The crystal oscillator is used in various electronic apparatuses as a reference source for frequency and time. One of such crystal oscillators is an oven-controlled crystal oscillator that maintains operating temperature of the crystal unit constant. Since the operating temperature of the crystal unit is maintained constant regardless of ambient temperature, the oven-controlled crystal oscillator offers particularly high frequency stability, and thus exhibits; for example, a frequency deviation of at most about 0.05 ppm. Such an oven-controlled crystal oscillator is used in, for example, communication facilities such as base stations for optical communication To maintain the crystal unit in the oven-controlled crystal oscillator at a constant temperature, a thermostatic chamber is generally used. The oven-controlled crystal oscillator is also called a crystal oscillator with a thermostatic chamber. In recent years, with increasing miniaturization of the communication facilities, the oven-controlled crystal oscillator has needed to be small. To miniaturize the oven-controlled crystal oscillator, for example, U.S. Pat. No. 7,345,552 and U.S. Pat. No. 7,382,204 disclose the use of a surface-mount type crystal unit as a crystal unit incorporated into the oven-controlled crystal oscillator. The surface-mount type crystal unit has a configuration suitable for surface-mounting on a circuit board and is characterized by a significantly small size compared to the sizes of conventional crystal units with leads
FIG. 1A is a sectional view showing an example of a configuration of a conventional oven-controlled crystal oscillator that uses a surface-mount type crystal unit. FIG. 1B is a bottom view of a surface-mount type crystal unit used in the crystal oscillator. FIG. 1C is a circuit diagram of a temperature control circuit in the crystal oscillator.
The illustrated oven-controlled crystal oscillator includes surface-mount type crystal unit 1, a plurality of oscillating elements 2 forming an oscillation circuit, and temperature control circuit 3 that maintains the operating temperature of surface-mount type crystal unit 1 constant. Crystal unit 1, oscillating elements 2, and temperature control circuit 3 are arranged on circuit board 4 and hermetically encapsulated in metal container 5 together with circuit board 4. Crystal unit 1 uses flat, substantially parallelepipedic container body 6 made up of ceramics. Crystal unit 1 includes quartz crystal blank 1A accommodated in a recess formed on one principal surface of container body 6 and closed by metal cover 7. Crystal blank 1A is secured to an inner bottom surface of the recess and hermetically encapsulated in container body 6. Mounting terminals used for surface-mounting the crystal unit on a wiring board are formed, as electrode layers with a rectangular planar shape, in four corners of an outer bottom surface of container body 6, that is, a principal surface of container body 6 on which no recess is formed. Of the four mounting terminals, two mounting terminals positioned at opposite ends of one diagonal line of the outer bottom surface of container body 6 are connection terminals 8a electrically connected to a pair of excitation electrodes (not shown) of crystal blank 1A. The remaining two mounting terminals are dummy terminals 8b normally electrically connected to metal cover 7 via via-holes (not shown) formed in container body 6. Dummy terminals 8b can be connected to, for example, a ground potential.
Temperature control circuit 3 maintains the operating temperature of crystal unit 1 constant, and includes at least heating chip resistor 3a, temperature sensitive resistor 3b that detects operation temperature of crystal unit 1, and power transistor 3c. For example, a thermistor with a resistance value decreasing with increasing temperature is used as temperature sensitive resistor 3b. Power transistor 3c supplies heating chip resistor 3a with power controlled by the resistance value of temperature sensitive resistor 3b which varies with temperature.
Specifically, as shown in FIG. 1C, temperature control circuit 3 includes differential amplifier 12 with its output connected to a base of power transistor 3c, and heating chip resistor 3a is interposed between a collector of transistor 3c and direct-current power supply PC. FIG. 1C shows only one chip resistor 3a. However, for example, two chip resistors 3a are provided in parallel for uniform heat conduction. An emitter of transistor 3c is grounded. Temperature sensitive resistor 3b and resistor Ra are provided in series between power supply DC and the ground point. When a voltage appearing at a connection point between temperature sensitive resistor 3b and resistor Ra is defined as a temperature sensitive voltage, the temperature sensitive voltage is supplied to one input terminal of differential amplifier 12. Furthermore, resistors Rb, Rc are provided in series between power supply DC and the ground point to divide a power supply voltage to generate a reference voltage. The reference voltage is supplied to the other input terminal of differential amplifier 12. In this configuration, the base of power transistor 3c is supplied with a voltage corresponding to a differential voltage between the temperature sensitive voltage, which depends on temperature, and the constant reference voltage. Chip resistor 3a is thus supplied with power from power supply DC. As a result, the power supplied to chip resistor 3a is controlled according to temperature measured by temperature sensitive resistor 3b to maintain the operating temperature of crystal unit 1 constant.
Circuit board 4 is made up of first board 4a and second board 4b held on first board 4a with a plurality of metal pins 9. Metal pins 9 include a function of electrically connecting second board 4b to first board 4a. First board 4a is made up of a glass-epoxy wiring board. Circuit elements 2x are mounted on a lower surface of first board 4a. Here, circuit elements 2x are circuit elements which make up the crystal oscillator but are other the crystal unit 1, adjustable element 2A, heating chip resistor 3a, temperature sensitive resistor 3b, and power transistor 3c. 
Second board 4b is made up of a ceramic wiring board and includes crystal unit 1 provided on an upper surface thereof by surface-mounting. Circuit elements which make up temperature control circuit but are other than power transistor 3c are provided on a lower surface of second board 4b. Specifically, heating chip resistor 3a and temperature sensitive resistor 3b are provided on the lower surface of second board 4b. 
Second board 4b is positioned above first board 4a. Silicone-based conductive resin 10 is applied to between first board 4a and second board 4b so as to cover heating chip resistor 3a and temperature sensitive resistor 3b. Power transistor 3c is a circuit element with a large height dimension and is thus provided on the upper surface of second board 4b closer to an end thereof.
Metal container 5 is made up of metal base 5a and metal cover 5b. Air-tight terminals 11 are provided which penetrate metal base 5a to hold first board 4a. Cover 5b is joined to metal base 5a by resistance welding. Thus, first and second boards 4a, 4b and the circuit elements mounted on the boards are hermetically encapsulated in the metal container.
To manufacture such an oven-controlled crystal oscillator, first, circuit board 4 with the circuit elements mounted thereon is placed on air-tight terminals 11 so as to be held thereon. Then, frequency-temperature characteristics of crystal unit 1 are individually measured before metal cover 5b is connected to metal base 5a. In general, the frequency-temperature characteristics of crystal unit 1 are expressed by a cubic curve for temperature in which a high temperature-side extremal corresponds to the minimum value, whereas a low temperature-side extremal corresponds to the maximum value. Thus, a measured temperature offering the high temperature-side local minimal value, for example, 80° C. is set to be the operating temperature of the crystal unit. Resistor Ra in temperature control circuit 3 is adjusted so that temperature control circuit 3 can keep the temperature of the crystal unit equal to the operating temperature. A adjusting capacitor (not shown) in the oscillation circuit matches oscillation frequency f with a nominal frequency.
Elements such as resistor Ra and the adjusting capacitor which require replacement and adjustment are provided, for example, in an outer peripheral portion of second board 4b as adjustable elements 2A.
In the above-described oven-controlled crystal oscillator, temperature control circuit 3, including heating chip resistor 3a, and crystal unit 1 are provided on second board 4b, composed of ceramics, which offers a high heat conductivity. This enables an increase in the efficiency of heat transmission between temperature control circuit 3 and crystal unit 1. First board 4a is composed of a glass-epoxy wiring board, offering a low heat conductivity. This enables heat dispersion to be suppressed. In this manner, a thermostatic chamber structure with a high energy utilization efficiency can be provided, and the oven-controlled crystal oscillator can be miniaturized.
In the oven-controlled crystal oscillator, heating chip resistor 3a and temperature sensitive resistor 3b of a temperature control circuit are provided on a principal surface of second board 4b which lies opposite a principal surface thereof on which crystal unit 1 is provided. Heat from chip resistor 3a travels to crystal unit 1 via second board 4b and a gap between the bottom surface of crystal unit 1 and second board 4b. The heat then reaches crystal blank 1A. On the other hand, on the lower surface of second board 4b, temperature sensitive resistor 3b is provided in proximity to heating chip resistor 3a. Thus, heat generated by chip resistor 3a travels earlier to temperature sensitive resistor 3b than to crystal unit 1. From a transient viewpoint, temperature sensitive resistor 3b detects the heating temperature of chip resistor 3a rather than the temperature of crystal unit 1. As a result, at the time of a change in temperature, the temperature of crystal unit 1 changes depending on the heating temperature of heating chip resistor 3a rather than becoming equal to a preset operating temperature. This prevents the temperature control of the crystal unit from properly following a change in temperature.
U.S. Pat. No. 7,382,204 proposes a configuration in which dummy terminal 8b of crystal unit 1 is connected to temperature sensitive resistor 3b in order to allow the temperature of crystal unit 1 to be easily transmitted to temperature sensitive resistor 3b. For example, as shown in FIG. 1C, dummy terminal 8b is electrically connected to the connection point between temperature sensitive resistor 3b and resistor Ra. The temperature of crystal unit 1 is transmitted to temperature sensitive resistor 3b via a conductive path between the connection point and dummy terminal 8b. Temperature sensitive resistor 3b can thus detect the actual temperature of the crystal unit in real time, and the temperature control can properly follow a change in temperature. In this case, dummy terminal 8b is not grounded. However, heat from heating chip resistor 3a travels to crystal unit 1 via second board 4b and the gap. This may delay arrival of the temperature of crystal unit 1 in connection with the temperature set by temperature control circuit 3. Disadvantageously, the temperature control is still prevented from properly following a change in ambient temperature.
Moreover, in the above-described configuration, the surface-mount type crystal unit is mounted on the upper surface of the second board, and the temperature control circuit is located on the lower surface thereof. Thus, reducing the height dimension of the crystal oscillator is difficult.
As a technique relating to the present invention, Japanese Patent Laid-Open Application Nos. 2001-127579 and 2001-308640 (JP-2001-127579A and JP-2001-308640A) disclose crystal oscillators in which a thick-film thermistor element is provided on a surface of a wiring board as a temperature sensitive resistor, and a crystal unit is surface-mounted on the wiring board so as to cover the thick-film thermistor element, thus allowing the temperature of the crystal unit to be accurately sensed and enabling a reduction in height dimension. JP-2001-127579A further discloses a use of a thick-film resistor provided on an outer surface of a container for the crystal unit as a heater for heating the crystal unit.